The Fischer-Tropsch (FT, below) synthesis is a technique developed by German chemists Franz Fischer and Hans Tropsch in 1920s, which produces synthetic fuel (hydrocarbons) from synthetic gas (hydrogen and carbon monoxide) according to the following reaction:(2n+1)H2+nCO→CnH(2n+2)+nH2O
Catalysts containing cobalt or iron are primarily used in this Fischer-Tropsch synthesis, and reaction conditions such as reaction temperature, reaction pressure, gas composition, and the like are determined based on the type of catalyst to be applied.
The Fischer-Tropsch synthesis can be largely classified depending on the reaction temperature into low temperature Fischer-Tropsch (LTFT) of which reaction is carried out at a temperature range between 200 and 250° C., and high-temperature Fischer-Tropsch (HTFT) of which reaction is carried out at a temperature range between 300 and 350° C. (Andrei Y. Khodakov et al, Chem. Rev., 2007, 107, 1672).
Traditionally, iron-based catalysts are mainly used in the high-temperature FT reaction which is advantageous for the synthesis of C2˜C4 light olefin and gasoline-range hydrocarbon products. Since the iron-based catalysts also show activity to the water gas shift reaction, they can be used with various compositions in which a synthetic gas ratio of hydrogen to carbon monoxide varies between 1 and 2. Furthermore, it shows the merit of being used even in the presence of carbon dioxide, a gas impurity.
Industrially, the iron-based catalysts have been applied in commercial FT processes such as CTL (coal-to-liquid) process for preparing a liquid hydrocarbon compound from a raw material of coal as they are inexpensive and strongly tolerant to sulfur-containing compounds. A representative example of the commercial processes includes the synthol process, which uses iron catalysts made of fused Fe produced from Sasol Limited.
The iron-based catalysts have been known to comprise iron carbide as the main active species, and on many researches for increasing the amount of iron carbide in the catalyst have been mostly made to improve the catalyst performance. However, since the structures of carbide/oxide (hydroxide) are formed in a complicated manner in the iron-based catalyst under the active conditions, there is a limit in improving the catalyst performance simply by the earlier researches of increasing the amount of iron carbide in the catalyst.
Meanwhile, if the iron particle is loaded on silica through a wetness impregnation method, it is difficult to uniformly load them in a high concentration of 20 wt % or more. Also, if the iron particle is obtained through a co-precipitation method, there occurs the problem that the particle is large and irregular.
In addition, if metal/silica catalysts are prepared by the earlier co-precipitation or wetness impregnation method, it gives such demerits that the particle size becomes larger and more irregular due to the agglomeration of particles when the metal content is increased, and that sintering may easily occur during calcination at a high temperature range between 400 and 600° C.
On the other hand, in order to design a compact reactor, it is more advantageous that the content of active metal in the supported catalyst becomes higher. In particular, in such a case of Fischer-Tropsch synthesis, the high loading amount of particle and the stability at a high temperature of 300° C. or more are required.
For securing such a thermal stability, hybrid structures between the active metal substances and porous silica which is recently used as a support in a supported catalyst have been developed, and such approaches for various structures like a core-shell structure, etc. have been attempted (Park et al., J. Mater. Chem., 2010, 20, 1239-1246).
Recently, Korean Patent Laid-open Publication No. 10-2013-0045024 discloses a method of manufacturing a metal/silica catalyst support via a branched metal silicate structure. When this catalyst is used in a high-temperature catalytic reaction, the active metal may be loaded in a high amount, and its thermal stability is high, thereby effectively providing a high performance. However, it also gives such disadvantages as complicated synthesis and long manufacturing time to make a mass production thereby difficult.